all the specs are in the fsm. youre battery is running a 12 volt system, so your battery terminals alone with motor off should be 12-13v. your alternator should produce at least an extra 2v charge, so with your motor on, your battery terminals should be 14-15v. this is a good operating range, so its not to say if you have 15.1v that something is wrong, it maybe just fine. just make sure your terminals are tight, and you have good ground contact for the negative battery ground.
EDIT: dont forget to turn load switches off. like ac off, headlights off, radio off, everything off, etc. extensive load on switches (like ac) can give a different reading at the battery due to the load being pulled from the system.
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